1. Technical Field
The present invention relates to a refrigerant compressor, and more particularly, to a slant plate type compressor, such as a wobble plate type compressor, with a variable displacement mechanism suitable for use in an automotive air conditioning system.
2. Description Of The Prior Art
It has been recognized that it is desirable to provide a slant plate type piston compressor with a displacement or capacity adjusting mechanism to control the compression ratio in response to demand. As disclosed in U.S. Pat. No. 4,428,718, the compression ratio may be controlled by changing the slant angle of the sloping surface of a slant plate in response to the operation of a valve control mechanism. The slant angle of the slant plate is adjusted to maintain a constant suction pressure in response to a change in the heat load of the evaporator of an external circuit including the compressor or a change in rotation speed of the compressor.
In an air conditioning system, a pipe member connects the outlet of an evaporator to the suction chamber of the compressor. Accordingly, a pressure loss occurs between the suction chamber and the outlet of the evaporator which is directly proportional to the "suction flow rate" therebetween as shown in FIG. 5. As a result, when the capacity of the compressor is adjusted to maintain a constant suction chamber pressure in response to appropriate changes in the heat load of the evaporator or the rotation speed of the compressor, the pressure at the evaporator outlet increases. This increase in the evaporator outlet pressure results in an undesirable decrease in the heat exchange ability of the evaporator.
Above mentioned U.S. Pat. No. 4,428,718 discloses a valve control mechanism, to eliminate this problem. The valve control mechanism, which is responsive to both suction and discharge pressures, provides controlled communication of both suction and discharge fluid with the compressor crank chamber and thereby controls compressor displacement. The compressor control point for displacement change is shifted to maintain a nearly constant pressure at the evaporator outlet portion by means of this compressor displacement control. The valve control mechanism makes use of the fact that the discharge pressure of the compressor is roughly directly proportional to the suction flow rate.
However, in the above-mentioned valve control mechanism, a single movable valve member, formed of a number of parts, is used to control the flow of fluid both between the discharge chamber and the crankcase chamber, and between the crankcase chamber and the suction chamber. Thus, extreme precision is required in the formation of each part and in the assembly of the large number of parts into the control mechanism in order to assure that the valve control mechanism operates properly. Furthermore, when the heat load of the evaporator or the rotation speed of the compressor is changed quickly, discharge chamber pressure increases and an excessive amount of discharge gas flows into the crank chamber from the discharge chamber through a communication passage of the valve control mechanism due to a lag time to such the action between the operation of the valve control mechanism and the response of the external circuit including the compressor. As a result of the excessive amount of discharge gas flow, a decrease in compression efficiency of the compressor, and a decline of durability of the compressor internal parts, occurs.
The variable displacement control mechanism in a slant plate type of compressor, in accordance with the present invention, was developed to take advantage of the relationship between discharge pressure and suction flow rate in a manner which overcomes the disadvantages of a prior art mechanism such as disclosed in the '718 patent. That is, the control mechanism of the present invention was designed to have a simple physical structure and to operate in a direct manner on a valve controlling element in response to discharge pressure changes, thereby resolving the complexity, excessive discharge flow and slow response time problems.
The '718 patent discloses a capacity adjusting mechanism used in a wobble plate type compressor. As is typical in this type of compressor, the wobble plate is disposed at a slant or incline angle relative to the drive axis, nutates but does not rotate, and drivingly couples the pistons to the drive source. This type of capacity adjusting mechanism, using selective fluid communication between the crank chamber and the suction chamber, however, can be used in any type of compressor which uses a slanted plate or surface in the drive mechanism. For example, U.S. Pat. No. 4,664,604, issued to Terauchi, discloses this type of capacity adjusting mechanism in a swash plate type compressor. The swash plate, like the wobble plate, is disposed at a slant angle and drivingly couples the pistons to the drive source. However, while the wobble plate only nutates, the swash plate both nutates and rotates. The term slant plate type compressor will therefore be used therein to refer to any type of compressor, including wobble and swash plate types, which use a slanted plate or surface in the drive mechanism.